The disclosure of Japanese Patent Application No. 2003-207751 filed Aug. 18, 2003 including specification, drawings and claims is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a gallium nitride-based semiconductor light emitting device and, more particularly, to a gallium nitride-based semiconductor light emitting device that has a high quality of semiconductor layer having less dislocation and that has a high level of luminous efficiency.
2. Description of the Related Art
A III group nitride-based compound is a direct transition type semiconductor whose stable phase is of a wurtzite structure and whose forbidden band width can be varied from 6.2 eV in case of AlN to 1.9 eV in case of InN. For this reason, attention has been drawn toward it as a material for light emitting device use that is available for the emission of a light from a visible shorter wavelength range to a near ultraviolet range. Under this circumstance, a gallium nitride-based semiconductor light emitting device that is comprised of a III group nitride-based compound has been being developed.
Of these III group nitride-based compounds, an AlGaInN-based compound that is expressed as the general formula of AlxGayIn1−x−yN (where 1≧x≧0, 1≧y≧0, and 1≧x+y≧0) has gone on being developed as a material for light emitting device and photo detector use for visible light because the wavelength of light that is emitted therefrom can be varied from ultraviolet rays to red color light rays. Especially, with it acting as a motive that there have been realized blue/green high luminance light emitting diodes each of that is based on using a gallium nitride (GaN)-based compound, further researches and studies on the material have been made. Also, since an AlGaN-based compound in which in the general formula above the variables x and y have been set as having a relation of x+y=1 is a semiconductor which is stable even at a high temperature of 500° C. or more, it has gone on being developed as a material for device use that is usable in a high-temperature environment, or is cooling unneeded.
Here, a general method that manufactures a semiconductor light emitting device using a III group nitride-based compound that is expressed as the general formula of AlxGayIn1-x-yN is using a single crystal of sapphire as a crystal substrate, causing various GaN-based crystal layers to be epitaxially grown on that substrate via buffer layers, and using a desired one of the GaN-based crystal layers as a relevant luminous portion. The reason for adopting that method is that, of the compounds expressed as the general formula of AlxGayIn1−x−yN (where 1≧x≧0, 1≧y≧0, and 1≧x+y≧0), a material of GaN has extreme difficulty synthesize the bulk crystal.
However, the difference between the sapphire substrate and the GaN material in terms of lattice constant is as great as approximately 16 percent. The defect density in the layer that has been grown amounts even to 106 to 109 cm−3. Within the GaN-based crystal layer that has been grown using that method, there exist with a high density the dislocations that are attributable to the non-alignment of lattices that occurs between the crystal layer and the sapphire substrate.
Namely, the sapphire substrate and GaN material are different in physical property, such as not only in lattice constant but also in thermal expansion coefficient. Therefore, the crystal defects that are called “the dislocations” occur in large number. The dislocations are inherited in the growth direction even when the GaN-based crystal has grown and the thickness of the layer has become increased. They then become a continuous portion of defects that is called “the dislocation line (through dislocation)” and it causes an impairment of the device propertu such as decreasing the life of the buleish purple laser.
The reason why the device operates even with such a high defect density is due to the nature, peculiar to the semiconductor made using a III group nitride-based compound, that the luminous efficiency does not largely decrease even if the defect density is high. However, for obtaining a high quality and high reliability of devices, decreasing the defect density is indispensable. As one method for decreasing the defect density there is the one that obtains a low-dislocation GaN-based crystal by using a mask layer (for example, see Japanese Patent Application Laid-Open No. 2000-91253.). According to this method, in the process of growing as a semiconductor layer the dislocation laterally flows if that layer has a thickness that is somewhat great. This results in that the semiconductor layer with lesser dislocations is formed.
Japanese Patent Application Laid-Open No. 2000-91253 is hereby incorporated by reference.